of the study in the third book of their series (35), were interested in the effects of spacing on vocabulary as well as upon intelligence ("attainment") tests in general.
In order to investigate vocabulary, they limited their consideration to children of middle-class families in which verbal stimulation, when present, would be at a high level and living conditions would deteriorate relatively little even when births were closely spaced. Upon analyzing the data, they found that
in each size of middle-class family, the vocabulary scores of the children are relatively high when births are widely spaced and relatively low when they are close together. (35, p. 126)
They also noted that although differences in nonverbal test scores were slight, the vocabulary scores decreased in the test at age 8 as the number of other young children in the families increased. In the same children at age 15 a similar fall was observed in the reading test scores, which are obviously related to vocabulary content.
They also examined the effect of interval on general attainment. Pointing out that in three-child families there are twenty-four possible combinations of sex and rank even before birth spacing is considered, he limited his analysis to the data from two-child families. On comparing test scores for three interval groupsó2 years or less, 2 to 4 years, and more than 4 years they found
. . . remarkably consistent differences in which the highest scores, at each age, are made by those with medium birth intervals. There is no evidence however that those with medium birth intervals increase their lead in the attainment tests between 8 and 15 years, and the effect of birth spacing on performance seems to be fully established by the age of 8, when they were first tested. (35, p. 132)
The Effects on Mothers
Earlier comments on the "maternal depletion syndrome" (14) should apply to mothers whose pregnancies are too close together as readily as to those whose pregnancies are too numerous. Short intervals between pregnancies would provide too little time for recovery, especially among women on diets that are only marginally adequate. Common sense and clinical impressions aside, however, the evidence is extremely scanty.
Eastman, in his Johns Hopkins study (62), has provided the only data available. He examined five factors in relation to birth interval: maternal anemia during pregnancy, toxemia of pregnancy, postpartum hemorrhage, puerperal fever, and maternal mortality. Anemia (hemoglobin less than 10 grams per 100 cc of blood) was more common (34.5 percent) in the "very brief" group (interval less than 12 months), but he considered the group too